There is a pressing need in a number of applications for optical light detectors which can register a response at the level of individual photons. Single photon detectors are threshold devices which detect the presence of 1 or more photons on the device, but cannot determine the number of photons. They are used for general low light level detection, as well as for various applications based around determining the arrival time of the photon at the detector.
The applications of single photon detectors include industrial inspection, environmental monitoring, testing of fibre optic cables and components, medical imaging, chemical analysis and scientific research. Many of these applications use the ability of a single photon detector to measure the arrival time of a single photon. In industrial inspection systems a bright laser pulse is directed at the object under inspection and the time for single photons from the pulse to be reflected are recorded. From the time of flight data it is possible to build a 3D image of the object. Similar techniques involving single photon detectors are used to determine the location of faults in optical fibres and components, and to measure particles in the atmosphere.
Single photon detection is also used in various forms of x-ray and radioisotrope imaging in medical imaging, as well as in laser optical imaging at infra-red wavelengths. Lifetime fluorescence measurements using single photon detection can be used in the diagnosis of some medical conditions. It is employed in analytical chemistry for determining the chemical recipe of a sample. Single photon detection is also used in scientific research in the field of particle physics, astrophysics and materials science.
Photon number resolving detectors, not only detect the presence of photons, but are able also to count the number of photons in an incident light pulse. Like single photon detectors they are able to determine the arrival time of the photons at the detector.
Photon number resolving detectors are required for low noise light detection based on photon counting. Here they have the advantage over single photon detectors that they can operate with higher light intensities.
The ability to resolve the number of photons in the incident pulse is also very important for many applications in quantum information technology. In a quantum relay, for example, it is necessary to distinguish between 0-, 1- and 2-photon detection events in each detector. A similar detector capability is needed for many of the gates used in linear optics quantum computing.
A photon number resolving detector which can operate at visible/near infra-red wavelengths (300-1100 nm) has applications for linear optics quantum computing, quantum relays and repeaters, quantum cryptography, photon number state generation and conditioning, and characterisation of photon emission statistics of light sources.
Currently Geiger Mode Silicon Avalanche Photodiodes (APDs) are used for low noise light detection at visible/near infra-red wavelengths (300-1100 nm).